1. Field of the Invention
The present invention relates to an image processing apparatus, an image processing method and a storage medium.
2. Related Background Art
There have conventionally been executed developments for multiplexing, in the image information, other information related with the image.
Recently there is being standardized so-called digital watermarking technology, for multiplexing, in image information such as a photograph or a picture, additional information such as the name of author thereof or whether the use thereof is permitted, in a visually not easily recognizable manner, for communication through a network such as internet.
For another application, with progress in the image quality of the image output apparatus such as a copying apparatus or a printer, it is required to prevent illegal forging of banknotes, stamps, valuable papers etc., there is developed a technology of embedding additional information in the image, in order to identify the output apparatus and the machine number of such apparatus from the image outputted on a sheet of paper.
For embedding the additional information in the image, there is conventionally known a technology of multiplexing the information by embedding the additional information in a high frequency region of a color difference component or a saturation component of low visual sensitivity in the image.
However, the aforementioned technologies are associated with the following drawbacks. FIG. 20 shows the mode of embedding of ordinary additional information in the digital watermarking technology, wherein image information A and additional information B are multiplexed through an adding unit 2001 into a multiplexed information C.
FIG. 20 shows an example of multiplexing the additional information in the real space area of the image information. If the multiplexed information C can be communicated without image processing such as various filterings or encoding such as irreversible encoding, the additional information B can be easily decoded from the multiplexed information B even in the conventional technology. The image information communicated in the internet can be decoded, if such information has certain noise resistance, even after passing a digital filter for image quality improvement such as edge enhancement or smoothing.
However, let us assume a case of printing the multiplexed image by an output apparatus such as a printer, and extracting the additional information from the obtained print. Also it is assumed that the printer to be used has a representing ability of two to several gradation levels per single color. In the recent ink jet printing, there are commercially available the apparatus capable of representing several gradation levels per single color by utilizing inks of low dye concentration or variably controlling the output dot diameter, but the gradation of the photographic image cannot be expected unless a pseudo gradation process is adopted. Thus, in the aforementioned assumption of outputting the multiplexed image utilizing the digital watermarking technology shown in FIG. 20 by a printer, the multiplexed information C is changed into quantized information D by a pseudo gradation process 2101 as shown in FIG. 21, and is further changed into significantly deteriorated on-paper information (print) E by printing on paper by a printer 2102. Therefore, the decoding of the additional information from the information on paper for the aforementioned purpose of forging prevention corresponds to the decoding of the additional information B from the on-paper information E after a series of processes shown in FIG. 21. The amount of information change by the processes 2101, 2102 is very large, so that it is very difficult to multiplex the additional information in a visually unrecognizable manner and to correctly decode such multiplexed additional information from the information on the paper.
On the other hand, FIG. 22 shows an example of a conventional digital watermarking technology of converting the image information in a frequency area for example by Fourrier transformation, instead of the real space area and executing synthesis for example in a high frequency region. Referring to FIG. 22, the image information is transformed into a frequency area by an orthogonal transformation process 2201, and the additional information is added by an adding device 2202 in a specified frequency which is visually not easily recognizable. After returning to the real space area by a reserver orthogonal transformation process 2203, there are passed filters involving major changes of pseudo gradation process and printer output, as in the case shown in FIG. 21.
FIG. 23 shows the procedure of separation of the additional information from the on-paper information. The information of the print is entered by passing the print through an image reading device 2301 such as scanner. The entered information, having gradational representation by a pseudo gradation process, is subjected to a reconstruction process 2302 which is a reverse pseudo gradation process. The reconstruction process is generally conducted with a LPF (low-pass filter). After the reconstructed information is subjected to an orthogonal transformation process 2303, the embedded information is separated in separation means 2304, from the electric power of a specified frequency.
As will be apparent from FIGS. 22 and 23, many complex processes are involved from the multiplexing of the additional information to the separation thereof. In case of a color image, a color conversion process for converting into the colors specific to the printer is also included in such series of processes. In order to achieve satisfactory separation even after such complex processes, it is necessary to introduce a signal of very high resistance. It is difficult to introduce a signal of high resistance while maintaining satisfactory image quality. Also the presence of plural complex process steps leads to a very long process time required for multiplexing and for separation.
Also, in the above-described conventional technologies, the information is added to the high frequency region, but, if the error diffusion method is executed in the pseudo gradation process of the later stage, the band area of the additional information may be mixed in the band area of the texture generated by error diffusion because of the characteristics of the high pass filter specific to the error diffusion method, whereby the failure of decoding becomes highly probable. Also, a scanner device of a very high precision is required for decoding. Thus the process shown in FIGS. 21 and 22 is not suitable if the pseudo gradation process is to be employed. Stated differently, there is required a multiplexing method for the additional information, exploiting the features of the pseudo gradation process.
As an example of combining the multiplexing of the additional information with the redundancy of the pseudo gradation process, there is known a conventional technology to be explained in the following.
At first, there is known a method of mixing data into the image signal by, in binary digitization with a systematic dither method, selecting either one of the dither matrixes representing a same gradation level. However, it is difficult to output the photographic high image quality by the systematic dither method unless there is employed a printer of a high resolution with a very excellent mechanical precision, because even a slight aberration in the mechanical precision results in a noise of low frequency such as lateral streaks which can be easily recognized on the paper.
Also a periodical change in the dither matrix disturbs the band of a specified frequency generated by the regularly arranged dither matrixes, thereby detrimentally affecting the image quality. Also the gradation representing ability differs significantly by the kind of the dither matrix. Particularly on paper, the change in the area rate resulting for example from overlapping of the dots varies depending on the dither matrixes, so that the density may be varied by the switching of the dither matrix even in an area where the signal represents a uniform density. Also in the decoding (separating) side, erroneous decoding is highly probable in the decoding method of estimating the dither matrix used in the binary digitization in a state where the pixel value of the image information, constituting the original signal, is unknown.
As another example, there is known a method of employing a color dither pattern and multiplexing the additional information by the arrangement of such dither pattern. Also in this method, the deterioration in image quality is unavoidable as in the foregoing method. Also in comparison with the foregoing method, though the additional information of a larger amount can be multiplexed, there will result a change in the color hue caused by the change in the arrangement of the color components, leading to a larger deterioration of the image quality particularly in a flat area. Furthermore, it is anticipated that the decoding on the paper becomes more difficult.
In any case, the aforementioned both methods utilizing the change in the dither matrix is associated with a drawback of difficulty in the decoding while the deterioration in the image quality is large.
Therefore, the present applicant already proposed a method of embedding codes by artificially preparing combinations of quantized values that are not generated in the ordinary pseudo gradation process, utilizing the texture generated by the error diffusion method.
This method does not visually deteriorate the image quality, since the shape of the texture is changed only microscopically. Also the density of the areal gradation can be maintained by a method of varying the quantizing threshold value for the error diffusion method, so that the multiplexing of a different signal can be achieved quite easily. However, in the above-proposed method, it is required, in the decoding side, to judge whether the texture is artificial or not. In the print on the paper, the texture may not be satisfactorily reproduced because of an aberration in the dot position from the desired ink landing position for example by a droplet deviation. Also in a color image, the multiplexing is principally conducted in a color component of lowest visual sensitivity, but the judgment of texture in the real space area is easily influenced by other color components, so that the separation of the multiplexed information becomes difficult.
Also for embedding a large amount of information such as audio information in an image, there is already known a conventional technology of converting the audio information into dot codes or so-called two-dimensional bar codes, and printing such codes in a marginal area of the image or in the image itself. In this method, however, the dot code and the image information do not constitute multiplexing of two different information, and the codes are not rendered unrecognizable visually. For reducing recognizability there is proposed a method of embedding the codes with a transparent paint in the image, but such method requires a special ink, thus leading to an increased cost, and the quality of the image outputted on paper is naturally deteriorated.